Semiconductor laser device having a resonator of a particular length for reducing threshold current density

ABSTRACT

A semiconductor laser device includes a substrate and a laminated structure formed on a top face of the substrate. The laminated structure includes (a) first and second guide layers and (b) a quantum well structure of compound semiconductor interposed between the first and second guide layers. The quantum well structure serves as a resonator of the device and includes at least one quantum well layer and at least one barrier layer. The quantum well layer has a thickness L z  and the barrier layer has the energy gap larger than the energy gap of the quantum well layer so as to form a energy difference V 0  between the bottom of the conduction band of the quantum well layer and the bottom of the conduction band of the barrier layer. The relationship represented by formula (I) is satisfied: 
     
         L.sub.z ≦h / 2 (2m*V.sub.0).sup.1/2                 (I) 
    
     wherein h is Planck&#39;s constant and m* is the effective mass of electrons within the quantum well layer.

This application is a continuation of U.S. application Ser. No.08/307,184, filed Nov. 17, 1994, U.S. Pat. No. 5,506,856 issued on Apr.9, 1996 which is a divisional application of U.S. application Ser. No.08/048,887 filed Apr. 15, 1993, U.S. Pat. No. 5,375,135 issued on Dec.20, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor lasers and, moreparticularly, to semiconductor lasers having a quantum well structurewhich can be used for fiber optic communication, information processingutilizing light, and the like.

2. Description of the Related Art

Semiconductor laser devices having a quantum well structure of compoundsemiconductor material are, based on theoretical studies, expected tohave excellent characteristics of performance such as a low thresholdvalue, high conversion efficiency and small temperature dependency.Recently, intensive efforts have been made for research and developmentof these devices.

A quantum well semiconductor laser device is generally known to have thefollowing structural features. A quantum well structure is interposedbetween a pair of guide layers and serves as a resonator of the laserdevice. The quantum well structure includes at least one quantum welllayer and at least one barrier layer. The thickness of the quantum welllayer is set sufficiently thin to generate quantum effect for electronsinjected into the quantum well structure.

The above-mentioned advantages of quantum well semiconductor laserdevices are based on the following theory. That is, because such adevice allows diffusion of electrons only in directions in twodimensions and thus forms quantum levels within the quantum well layer,changes in density of states of electrons would be more abrupt and gaindistribution generated by electrons would be more restricted than in thecase of semiconductor laser devices having conventional doubleheterostructures wherein electrons diffuse in directions in threedimensions. On the other hand, because holes have heavier mass and thussmaller energy differences among existing quantum levels compared withthe case of electrons, quantum effect is usually not significantlyattained for holes within the quantum well structure. Therefore,characteristics of a quantum well semiconductor laser device are mainlyaffected by density of states of electrons.

The above-mentioned abrupt changes in density of states of electrons areknown to be shown by electrons in all quantum levels, regardless of thenumber of quantum well layers and the length of the resonator of asemiconductor laser device. For this reason, the number of quantumlevels allowed to exist within the quantum well structure, as well asthe number of quantum well layers and the length of the resonator, haveattracted little or no specific attention in studies for manufacturingquantum well semiconductor laser devices.

However, conventional quantum well semiconductor laser devices have notshown as good characteristics as have been expected. For example,semiconductor laser devices for household machines, such as those usedfor fiber optic communication between a telephone central office and thehomes of subscribers, are often used under rigorous conditions and thusespecially require satisfactory performance characteristics at hightemperature. Nonetheless, conventional quantum well semiconductor laserdevices to date have not satisfactorily realized desired characteristicssuch as those enabling laser oscillation at significantly hightemperature while preventing saturation of optical output. Thisparticular problem is believed to be caused because of the followingreason. That is, as temperature of the semiconductor laser device risesand a threshold current density required for attaining laser oscillationincreases, carriers injected into a quantum well layer overflow to anadjacent barrier layer, thereby further accelerating increase in thethreshold current density.

To overcome the above-mentioned problem, attempts have been made to coatthe faces of the resonator of a quantum well semiconductor laser devicewith dielectric layers having high reflectivity. Such coating can reducethe threshold current density and thus enables laser oscillation athigher temperature. However, these semiconductor laser devices aredifficult to use for generating optical output of significantly highintensity.

Further, when used as a component in a fiber optic communication system,a semiconductor laser device is usually coupled with an optical fiberthrough a lens interposed therebetween. In order to improve transmissioncharacteristics of the system, the semiconductor laser device shouldhave high coupling efficiency with the optical fiber. It is known thatachievement of higher coupling efficiency makes it desirable for laserlight emitted from the semiconductor laser device to have a circularimage rather than an elliptic image along a plane parallel to thelight-emitting face of the device. Therefore, this desiredcharacteristic would preferably be incorporated into structural featuresof quantum well semiconductor laser devices.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a semiconductorlaser device having a resonator includes: a substrate; and a laminatedstructure formed on a top face of the substrate, the laminated structureincluding (a) first and second guide layers and (b) a quantum wellstructure of compound semiconductor interposed between the first andsecond guide layers, the quantum well structure serving as the resonatorand including at least one quantum well layer and at least one barrierlayer; and wherein the quantum well layer has a thickness L_(z) and thebarrier layer has the energy gap larger than the energy gap of thequantum well layer so as to form a energy difference V₀ between thebottom of the conduction band of the quantum well layer and the bottomof the conduction band of the barrier layer; and wherein therelationship represented by formula (I) is satisfied:

    L.sub.z ≦h / 2 (2m*V.sub.0).sup.1/2                 (I)

wherein h is Planck's constant and m* is the effective mass of electronswithin the quantum well layer.

In accordance with another aspect of the present invention, asemiconductor laser device having a resonator includes: a substrate; anda laminated structure formed on a top face of the substrate, thelaminated structure including (a) first and second guide layers and (b)a quantum well structure of compound semiconductor interposed betweenthe first and second guide layers, the quantum well structure serving asthe resonator and including a plurality of quantum well layers and aplurality of barrier layers stacked in alternating manner; and whereinthe number of the quantum well layers is in the range of 6 to 10, andthe length of the resonator may be in the range of 250 μm to 700 μm.

The invention described herein makes possible, among others, theadvantages of (1) improving temperature characteristics of a quantumwell semiconductor laser device by reducing overflow of carriers andobtaining sufficient optical output when the device is operated at hightemperature; and (2) enabling a quantum well semiconductor laser deviceto realize high coupling efficiency with an optical fiber and the like.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevation view illustrating an embodiment of asemiconductor laser device according to the present invention.

FIG. 2 is a conceptual view illustrating the energy band structure ofand around the quantum well structure of the semiconductor laser devicein FIG. 1.

FIG. 3 is a graph illustrating a preferred relationship between L_(z)and V₀ for semiconductor laser devices according to the presentinvention, wherein electrons have an effective mass of 0.05 m₀.

FIG. 4a is a graph illustrating current vs. optical outputcharacteristics of the semiconductor laser device of FIG. 1.

FIG. 4b is a graph illustrating current vs. optical outputcharacteristics of a conventional semiconductor laser device.

FIG. 5 is a graph illustrating a distribution of electrons at anexemplary high temperature condition which are injected into a quantumwell structure which allows for two quantum levels to exist therein.

FIG. 6 is a schematic perspective view illustrating another embodimentof a semiconductor laser device according to the present invention.

FIG. 7 is a conceptual view illustrating the energy band structure ofand around the quantum well structure of the semiconductor laser deviceof FIG. 6.

FIG. 8 is a graph illustrating variation of threshold current densitiesof semiconductor laser devices, as functions of the length of theresonator and the number of the quantum well layers, where thesemiconductor laser devices are made of InGaAsP/InP type materials andhave guide layers with a GRIN-SCH structure.

FIG. 9a is a schematic view illustrating an angle φ measured from theperpendicular line which is perpendicular to a light emitting face of asemiconductor laser device.

FIG. 9b is a graph illustrating a relationship between the angle φ andintensity of emitted laser light in view of determining the spreadingangle θ of laser light.

FIG. 10 is a graph illustrating relationships between the spreadingangle θ and the thickness of the guide layers for semiconductor laserdevices having varied number of quantum well layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A quantum well semiconductor laser device generally has a low thresholdcurrent density at around room temperature and thus oscillates laserlight even with a relatively small amount of injected carriers. Such alaser oscillation has a wavelength corresponding to the lowest quantumlevel (n=1) of electrons even where the quantum well structure allowsfor more than two quantum levels to exist therein. As the temperature ofthe semiconductor laser device rises and accordingly the thresholdcurrent density increases, however, injected carriers are also placed inthe second lowest quantum level (n=2) and laser oscillation begins witha wavelength corresponding to the second lowest quantum level. FIG. 5illustrates a distribution of electrons in such a situation (i.e., whenthe quantum well semiconductor laser device is operated at hightemperature). This distribution is obtained as a product of Fermidistribution function and density of states of electrons. As isrepresented in FIG. 5, under high temperature conditions there is anincreased number of electrons having higher energy, which would overflowfrom a quantum well layer to an adjacent barrier layer, thereby furtheraccelerating the increase of the threshold current density.

According to an embodiment of semiconductor laser device of thisinvention, a thickness L_(z) of a quantum well layer within the quantumwell structure and the energy difference V₀ between the bottom of theconduction band of the quantum well layer and the bottom of theconduction band of an adjacent barrier layer satisfy the relationshiprepresented by formula

    L.sub.z ≦h / 2 (2m*V.sub.0).sup.1/2                 (I)

wherein h is Planck's constant and m* is the effective mass of electronswithin the quantum well layer. When this relationship is satisfied, thequantum well structure allows only for the lowest quantum level (n=1) toexist therein. Therefore, even when operated at high temperature, asemiconductor laser device of this invention does not oscillate with awavelength corresponding to the second lowest quantum level, and thuscan restrict the overflow of carriers.

According to another embodiment of semiconductor laser device of thisinvention, the number of the quantum well layers within a quantum wellstructure is set in the range of 6 to 10, and the length of theresonator of the device is set in the range of 250 μm to 700 μm. Byselecting the parameters of semiconductor laser device in the specifiedrange, improved temperature characteristics of the device are obtainedand, at the same time, high coupling efficiency of the device withoptical fiber and the like is facilitated.

EXAMPLE 1

With reference to FIG. 1, a schematic elevation view of an embodiment ofa semiconductor laser device according to the present invention isillustrated.

The semiconductor laser device of FIG. 1 has a substrate 108 made ofn-type InP and a laminated structure formed on the top face of thesubstrate 108. The laminated structure includes, in the direction awayfrom the substrate 108 (i.e., vertically as viewed in the drawing), ann-cladding layer 101 made of n-type InP, a first guide layer 102 made ofInGaAsP (λ_(g) =1.1 micrometers (μm); thickness=150 nanometers (nm)), anactive layer 107 having a quantum well structure, a second guide layer105 made of InGaAsP (λ_(g) =1.1 μm; thickness=150 nm), a p-claddinglayer 106 made of p-type InP, and a p⁺ - cap layer 111 made of p⁺ -typeInGaAsP (λ_(g) =1.3 μm). Within the active layer 107, a plurality ofquantum well layers 103 each made of InGaAsP (λ_(g) =1.37 μm;thickness=8 nm), and a plurality of barrier layers 104 each made ofInGaAsP (λ_(g) =1.1 μm; thickness=12 nm) are stacked in alternatingmanner. λg is the constant corresponding to the band gap energy E_(g) ofeach semiconductor material, and is obtained according to the formula:E_(g) =hc/λ_(g), wherein h is Planck's constant and c is the speed oflight.

The above laminated structure is obtained by sequentially growing on thesubstrate 108 each of the layers 101, 102, 107, 105, 106 and 111 by useof conventional epitaxial growth method. The laminated structure is thensubjected to etching treatment to form a stripe-shaped portion 107'.This stripe-shaped portion is provided on both of its side faces with acurrent blocking layer including a p-type InP layer 109 and an n-typeInP layer 110 in a manner as shown in FIG. 1. An electrode 112 is formedon the bottom face of the substrate 108, and another electrode 113 isformed to be in contact with the p⁺ -cap layer 111 and the n-type InPlayer 110, thereby yielding the semiconductor laser device of thisexample.

FIG. 2 illustrates the energy band structure of and around the activelayer of the above semiconductor laser device.

An energy band of the portion 207 of the energy band structure in FIG. 2corresponds to the energy band of the active layer 107 in FIG. 1.Further in FIG. 2, energy levels 201, 202, 203, 204, 205 and 206respectively represent the bottoms of the conduction bands of then-cladding layer 101, the first guide layer 102, the quantum well layer103, the barrier layer 104, the second guide layer 105 and thep-cladding layer 106. The thickness of the quantum well layer 103 andthat of the barrier layer 104 are respectively shown as L_(z) and L_(b),and the energy difference between the energy level 203 of the quantumwell layer and the energy level 204 of the barrier layer is shown as V₀.

According to the present invention, the relationship between thethickness L_(z) and the energy difference V₀ preferably satisfies theformula (I) as illustrated above. The thickness L_(z) is set optionallybut should be sufficiently thin to attain quantum effect for electronsinjected into the quantum well layer 103. The thickness L_(b) is alsoset optionally but should be sufficiently thick to prevent the quantumwell layers 103, if more than two of them exist in the active layer 107,from being in contact with each other. The energy difference V₀ can bechanged as desired by changing the composition ratio of semiconductormaterial(s) used for the quantum well layer 103 and/or the barrier layer104, and does not depend on either of the thicknesses L_(z) and L_(b).In addition, compound semiconductor material(s) other than InGaAsP maybe used for the quantum well layer 103 and/or the barrier layer 104 aslong as desired energy difference V₀ can be obtained.

FIG. 3 illustrates a preferred relationship between L_(z) and V₀ forsemiconductor laser devices such as that shown in Example 1. That is,when a quantum well structure includes a quantum well layer made ofInGaAsP (λ_(g) =1.37 μm) and therefore the effective mass m* ofelectrons therein is represented as 0.05 m₀ (m₀ is the rest mass ofelectrons), the relationship of the above formula (I) is satisfied inthe shaded area shown in FIG. 3. Because each of the quantum well layers103 of the semiconductor laser device of Example 1 has a thickness L_(z)of 8 nm and the energy difference V₀ thereof is determined to be 52milli-electron volts (meV), the relationship of the formula (I) issatisfied. Thus, only the lowest quantum level (n=1) is allowed to existwithin the quantum well structure of this semiconductor laser device.

FIG. 4a illustrates current vs. optical output characteristics, measuredat 85° C., of the semiconductor laser device shown in Example 1. It canbe seen that this device is free from saturation of optical output evenat such a high temperature and shows excellent current vs. opticaloutput characteristics. FIG. 4b illustrates current vs. optical outputcharacteristics, also measured at 85° C., of a semiconductor laserdevice which has been prepared in the same manner as illustrated inExample 1 except that its quantum well layer has the thickness L_(z) of15 nm. It can be seen that this device suffers from significantsaturation of optical output and shows inferior characteristics comparedwith the device according to the present invention.

Although the first and second guide layers 102 and 105, respectively, ofthe semiconductor device of Example 1 have uniform composition ratioalong the transverse direction of each layer, these layers may have aso-called GRIN-SCH (Graded Index Separate Confinement Hetero) structurewherein composition ratio is changed in stepwise manner along thetransverse direction of the layers. In addition, the above semiconductordevice may have a single quantum well layer instead of a plurality ofquantum well layers illustrated above. When a quantum well structure hasonly a single quantum well layer, a barrier layer may be integrated intoa guide layer.

EXAMPLE 2

With reference to FIG. 6, a schematic perspective view of anotherembodiment of a semiconductor laser device according to the presentinvention is illustrated.

The semiconductor laser device of FIG. 6 has a substrate 608 made ofn-type InP and a laminated structure formed on the top face of thesubstrate 608. The laminated structure includes, in the direction awayfrom the substrate 608, an n-cladding layer 601 made of n-type InP, afirst guide layer 602 having a four-layer GRIN-SCH structure made ofInGaAsP (λ_(g) =0.95, 1.0, 1.05 and 1.1 μm for the respective layers inthe direction away from the n-cladding layer 601; thickness in total=100nm), an active layer 607 having a quantum well structure, a second guidelayer 605 having a four-layer GRIN-SCH structure made of InGaAsP (λ_(g)=1.1, 1.05, 1.0 and 0.95 μm for the respective layers in the directionaway from the active layer 607; thickness in total=100 nm), a p-claddinglayer 606 made of p-type InP, and a p⁺ -cap layer 611 made of p⁺ -typeInGaAsP (λ_(g) =1.3 μm). Within the active layer 607, a plurality ofquantum well layers 603 each made of InGaAsP (λ_(g) =1.37 μm;thickness=8 nm), and a plurality of barrier layers 604 each made ofInGaAsP (λ_(g) =1.1 μm; thickness=12 nm) are stacked in alternatingmanner. In this example, the active layer 607 has nine (9) quantum welllayers 603 and seven (7) barrier layers 604.

The above laminated structure is obtained by sequentially growing on thesubstrate 608 each of the layers 601, 602, 607, 605, 606 and 611 by useof conventional epitaxial growth method. The laminated structure is thensubjected to etching treatment to form a stripe-shaped portion 607'.This stripe-shaped portion is provided on both of its side faces with acurrent blocking layer including a p-type InP layer 609 and an n-typeInP layer 610 in a manner as shown in FIG. 6. An electrode 612 is formedon the bottom face of the substrate 608, and another electrode 613 isformed to be in contact with the p⁺ -cap layer 611 and the n-type InPlayer 610, thereby resulting in the semiconductor laser device of thisexample. In this semiconductor laser device, the active layer 607 whichserves as the resonator has the length L_(c) of 350 μm along thedirection of the resonator, as shown in FIG. 6.

FIG. 7 illustrates the energy band structure of and around the activelayer of the above semiconductor laser device.

An energy band of the portion 707 of the energy band structure in FIG. 7corresponds to the energy band of the active layer 607 in FIG. 6.Further in FIG. 7, energy bands 701, 702, 703, 704, 705 and 706respectively represent the energy bands of the n-cladding layer 601, thefirst guide layer 602, the quantum well layer 603, the barrier layer604, the second guide layer 605 and the p-cladding layer 606. Thestepwise changes shown in the energy bands 702 and 705 reflect theGRIN-SCH structures of the first guide layer 602 and the second guidelayer 605. However, the semiconductor laser device of this example mayinclude a guide layer having uniform composition ratio along thetransverse direction of the layer.

As mentioned above, the quantum well semiconductor laser device of thisexample has nine (9) quantum well layers and the resonator length of 350μm. However, advantages of this invention are most preferably realizedas long as a quantum well semiconductor laser device has the number ofthe quantum well layers in the range of 6 to 10, and the resonatorlength in the range of 250 μm to 700 μm. The reason for these specificranges being preferred will be described below.

FIG. 8 illustrates variation of threshold current densities as functionsof the length L_(c) of the resonator and the number N_(w) of the quantumwell layers, for semiconductor laser devices such as that obtained inExample 2 (i.e., quantum well semiconductor laser devices which are madeof InGaAsP/InP type materials and have a pair of guide layers eachhaving a GRIN-SCH structure). As a quantum well semiconductor laserdevice has larger number of quantum well layers, it can achieve largergain for a fixed amount of injected carriers and thus have a lowerthreshold current density for laser oscillation. However, when thenumber of quantum well layers increases beyond a certain point, effectof the increased volume of those quantum well layers dominates theeffect of the increased gain, and thus the threshold current densitybegins to increase again. In other words, a quantum well semiconductorlaser device including a resonator of a fixed length has an optimumnumber with regard to its quantum well layers, by which the lowestthreshold current density can be achieved. As can be seen from FIG. 8,this optimum number increases as the resonator length L_(c) decreases.This means that as a quantum well semiconductor laser device has largernumber of quantum well layers, smaller resonator length becomesappropriate to achieve the lowest threshold current density.

With regard to quantum well semiconductor laser devices each having avaried resonator length L_(c) and an optimum value for the number N_(w)of quantum well layers to achieve the lowest of its threshold currentdensity J_(th), the number N_(w) can be correlated respectively with thethreshold current I_(th), the differential quantum efficiency η_(d) andthe threshold current density J_(th) according to the following formulae(II) through (IV):

    I.sub.th =[C.sub.1 / (A.sub.1 N.sub.w -B.sub.1)]+D.sub.1   (II)

    η.sub.d =E.sub.2 / [C.sub.2 / (A.sub.2 N.sub.w -B.sub.2)+D.sub.2](III)

    J.sub.th / N.sub.w =A.sub.3                                (IV)

wherein A₁, B₁, C₁, D₁, A₂, B₂, C₂, D₂, E₂ and A₃ are constants.

It can be seen from the formulae (II) and (III) that as a quantum wellsemiconductor laser device has larger number of quantum well layers, ithas a decreased threshold current I_(th) and an increased differentialquantum efficiency η_(d).

On the other hand, when quantum well semiconductor laser devices eachhaving a fixed N_(w) value and a varied L_(c) value are considered,increase in the resonator length L_(c) leads to decrease in thethreshold current density J_(th) and thus to decrease in the thresholdcurrent density per each quantum well layer, which corresponds to A₃ informula (IV). Therefore, as a quantum well semiconductor laser devicehas a longer resonator length, it can better reduce the above-mentionedoverflow of injected carriers and thus shows more improved temperaturecharacteristics enabling higher optical output even at high temperature.

When a quantum well semiconductor laser device has an excessively longresonator, however, effect of internal heat evolution becomessignificant and thus the temperature within the device rises, resultingin deterioration of characteristics of the device. As described above, aquantum well semiconductor laser device having a smaller number ofquantum well layers requires a longer resonator length in order toachieve the lowest threshold current density. Therefore, in view ofreducing the undesired effect of heat evolution, a quantum wellsemiconductor device should have a larger number of quantum well layers,preferably more than 6, and more preferably more than 8.

On the other hand, a quantum well semiconductor laser device having alarger number of quantum well layers has a more enlarged spreading angleθ when the spreading angle is measured in the direction perpendicular tothe quantum well layers. Herein the spreading angle θ is determined inthe manner as described with respect to FIGS. 9a and 9b. In FIG. 9a, anangle φ is the angle measured from a perpendicular vector L which isperpendicular to a light emitting face 900 of a semiconductor laserdevice such as shown in Example 2, and which represents the principalpropagation direction of light emitted therefrom. A direction arrow Prepresents the direction perpendicular to the alignment of the quantumwell layers of the device. A direction arrow H represents the directionhorizontal to the alignment of the quantum well layers of the device.FIG. 9b illustrates a relationship between the angle φ and intensity ofthe emitted laser light. In FIG. 9b, I₀ represents the maximum intensityof the laser light and φ₋ and φ₊ represent the angles where intensity ofthe emitted laser light becomes one half of I₀. The spreading angle θ isdefined as the absolute value of (φ₊ -φ₋).

FIG. 10 illustrates relationships between the perpendicular spreadingangle θ (i.e., the spreading angle θ measured in the directionperpendicular to the quantum well layers, that is, in the planecontaining arrow P and vector L) and the thickness of the guide layersfor semiconductor laser devices having varied number of quantum welllayers. Data shown in this graph are obtained from semiconductor laserdevices which are made of InGaAsP/InP type materials and have first andsecond guide layers each having a GRIN-SCH structure and an equalthickness, as in the case of the device prepared in Example 2. As can beseen from FIG. 10, a larger number of quantum well layers is accompaniedby a more enlarged perpendicular spreading angle θ. On the other hand,the horizontal spreading angle θ (i.e., the spreading angle θ of aquantum well semiconductor laser device measured in the directionhorizontal to the quantum well layers, that is, in the plane containingarrow H and vector L) is generally known to be about 25°. Thus, byapproximating the perpendicular spreading angle θ to 25°, the ellipticratio (i.e., ratio of the perpendicular spreading angle to thehorizontal spreading angle) can be closer to one (1), thereby enablingthe semiconductor laser device to achieve higher coupling efficiencywith an optical fiber and the like. The perpendicular spreading angle θis preferably not more than 30°. Therefore, from FIG. 10, the number ofquantum well layers is preferably not more than 10.

It can be also seen from FIG. 10 that even when the number of quantumwell layers is small, such as N_(w) =4, an appropriate perpendicularspreading angle θ can be obtained by increasing the thicknesses of guidelayers. However, excessive increase in the thicknesses of the guidelayers is undesirable because it can result in reducing efficiency forinjecting carriers into the quantum well layer placed between theseguide layers. When a guide layer has a GRIN-SCH structure, thethicknesses thereof is preferably in the range of about 20 nm to about200 nm.

As has been discussed above, in order to realize a low threshold currentdensity and desirable temperature characteristics, and at the same timesecure the perpendicular spreading angle θ within an appropriate range,a semiconductor laser device of this invention has the number of quantumwell layers preferably in the range of 6 to 10. Also, in order torealize a low threshold current density and desirable temperaturecharacteristics, and at the same time restrict the undesired internalheat evolution, a semiconductor laser device of this invention has thelength of its resonator preferably in the range of 250 μm to 700 μm, andmore preferably in the range of 250 μm to 400 μm.

The semiconductor laser device obtained in Example 2, which has nine (9)quantum well layers and the resonator length of 350 μm, shows excellenttemperature characteristics at high temperature, as indicated by thecharacteristic temperature T₀ of 60 Kelvin(K), and provides opticaloutput of sufficiently high intensity. In addition, this device has theperpendicular spreading angle θ of 30°, which falls within theabove-mentioned desirable range.

As can be seen from the above examples, the present invention provides asemiconductor laser device which has improved temperaturecharacteristics and generates optical output of sufficiently highintensity even when operated at high temperature, and which can alsorealize high coupling efficiency with photo-receiving device such as anoptical fiber and the like.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor laser device having a resonator,comprising: a substrate; anda laminated structure formed on a top faceof the substrate, the laminated structure including (a) first and secondguide layers and (b) a quantum well structure of compound semiconductorinterposed between the first and second guide layers, the quantum wellstructure serving as the resonator and including a plurality of quantumwell layers and a plurality of barrier layers stacked in alternatingmanner; and wherein the number of the plurality of quantum well layersis in the range of 7 to 10, and the length of the resonator is in therange of 250 μm to 700 μm.
 2. The device according to claim 1, whereinthe first and second guide layers respectively have a Graded IndexSeparate Confinement Heterostructure.
 3. The device according to claim1, further comprising a current blocking layer, and wherein thelaminated structure has a stripe-shaped geometry and is placed incontact with the current blocking layer on the side faces of thestripe-shaped geometry.
 4. The device according to claim 1, wherein thethicknesses of the first and second guide layers are selected so as togive a laser light emitted from the device a perpendicular spreadingangle in the range of about 25° to about 30°.
 5. The device according toclaim 4, wherein said first and second guide layers each have athickness of less than 200 nm.
 6. The device according to claim 1 havinga threshold current density in the range of 500 A/cm² to 1500 A/cm². 7.The device according to claim 1 having a characteristic devicetemperature of about 60K.
 8. The device according to claim 1, whereinthe number of the plurality of quantum well layers is in the range of 8to
 10. 9. The device according to claim 1, wherein at least one of theplurality of quantum well layers has a thickness L_(z), wherein at leastone of the plurality of barrier layers has a first energy gap, and atleast one of the plurality of quantum well layers has a second energygap, and wherein said first energy gap is larger than said second energygap, as to form an energy difference V₀ between the bottom of theconduction band of the at least one of the plurality of quantum welllayers and the bottom Of the conduction band of the at least one of theplurality of barrier layers; and wherein the relationship represented byformula (I) is satisfied:

    L.sub.z ≦h/2 (2m* V.sub.0).sup.1/2

where h is a Planck's constant and m* is the effective mass of electronswithin the at least one of the plurality of quantum well layers.